1. Field of the Invention
The present invention relates to an external-electrode discharge lamp, and more particularly to an external-electrode discharge lamp for use as the light source of a backlight for a liquid crystal display device having a relatively large screen size and a method of manufacturing such an external-electrode discharge lamp.
2. Description of the Related Art
Discharge lamps are used as the light source of backlights for liquid crystal display devices in one of their applications. A backlight serves to illuminate a liquid crystal display panel from its back. From the standpoint of the layout of a discharge lamp as a light source, backlights are generally classified into a so-called edge-light type and a so-called direct type. The edge-light backlight design includes a discharge lamp as a light source disposed outwardly of an edge of the liquid crystal display panel. In the edge-light backlight design, light emitted from the discharge lamp is guided by a light guide plate to the back of the liquid crystal display panel. The direct-type backlight includes a discharge lamp as a light source disposed behind the liquid crystal display panel. In the direct-type backlight, light emitted from the discharge lamp is directly applied to the back of the liquid crystal display panel.
Liquid crystal display panels are used in various applications in terms of screen sizes. In some applications, e.g., cellular phone terminals and laptop personal computers, liquid crystal display panels that are used can be of a relatively small screen size. In other applications, e.g., liquid crystal television receivers, the screen sizes of liquid crystal display panels that are used may be preferably as large as possible. Liquid crystal display panels in the former applications where the screen sizes can be smaller are often combined with an edge-light backlight unit which employs a cold-cathode discharge lamp as a light source. On the other hand, liquid crystal display panels in the latter applications where the screen sizes should be greater are often combined with a direct-type backlight which has a light source comprising an external-electrode mercury fluorescent lamp.
Large-screen liquid crystal display panels are associated with a direct-type backlight for the following reasons: Liquid crystal television receivers that are required to have larger screen sizes need to have the screen luminance increased, and hence the backlight combined therewith is required to have increased luminance. For increasing the luminance of the backlight, the number of lamps used as light sources needs to be increased. If the number of lamps in the edge-light backlight is increased, then many lamps are stacked, resulting in an increase in the thickness of the display panel. If the number of lamps in the direct-type backlight is increased, then since many lamps are placed in a planar array, the thickness of the display panel remains unchanged though the number of lamps used is large. For this reason, the direct-type backlight is advantageously effective to keep large-screen liquid crystal display panels low in profile.
The backlight for large-screen liquid crystal display panels employs an external-electrode discharge lamp for the following reasons: Generally, discharge lamps exhibit negative current vs. voltage characteristics, which are corrected into positive current vs. voltage characteristics by a capacitor called a ballast capacitor. If the number of cold-cathode discharge lamps is increased, then the number of ballast capacitors also needs to be increased depending on the number of cold-cathode discharge lamps. As a result, a power supply for turning on the lamps becomes larger in size and more expensive to manufacture. The external-electrode discharge lamp has an outer casing such as a glass bulb acting as a ballast capacitor, and hence, so to say, has a capacitor itself. Carrying logic to extreme, it is not necessary for each external-electrode discharge lamp to have a dedicated ballast capacitor, and even if the number of external-electrode discharge lamps used is increased, a power supply for turning them on does not need to be substantially increased in size.
FIGS. 1A and 1B of the accompanying drawings show a basic structure of a conventional external-electrode discharge lamp. The conventional external-electrode discharge lamp is a mercury fluorescent lamp having cylindrical glass bulb 1. Glass bulb 1 is made of borosilicate glass, for example. The external-electrode discharge lamp has external electrodes 2A, 2B mounted on the respective outer surfaces of opposite ends of glass bulb 1, and they are electrically insulated from each other. Glass bulb 1 is hollow and has a closed space (discharge chamber) defined therein. The discharge chamber is filled with a gas serving as a discharge medium, e.g., a mixed gas of xenon and mercury vapor or a mixture of such a mixed gas and another rare gas such as argon, neon, or the like, under a pressure ranging from 1.3×103 to 40×103 Pa (10 through 300 Torr).
Basic components involved in developing an electric discharge are above-mentioned glass bulb 1, the gas as the discharge medium, and external electrodes 2A, 2B. In addition, the external-electrode discharge lamp also has fluorescent layer 4 disposed on the inner surface of glass bulb 1 which faces the discharge chamber. Fluorescent layer 4 serves to convert ultraviolet radiation produced in glass bulb 1 by an electric discharge into light having other wavelengths, such as visible light. Protective layers 3 are also disposed on respective portions of the inner surface of glass bulb 1 underneath external electrodes 2A, 2B. Protective layers 3 serve to protect the inner surface of glass bulb 1, and are made of a metal oxide such as yttrium oxide, for example.
The external-electrode discharge lamp thus constructed operates as follows: External power supply 15 applies AC electric power having a frequency ranging from 10 to 100 kHz and a voltage ranging from 1 to 10 kV, for example, between external electrodes 2A, 2B, generating a dielectric barrier discharge in the discharge chamber with the wall of glass bulb 1 serving as a dielectric. Ultraviolet radiation caused by the dielectric barrier discharge excites fluorescent layer 4. When excited, fluorescent layer 4 emits light. In this manner, the ultraviolet radiation is converted into light having other wavelengths. The converted light is radiated out of the external-electrode discharge lamp through glass bulb 1.
Heretofore, it is known that external electrodes 2A, 2B are made of metal foil such as aluminum foil or copper foil and bonded to the glass bulb 1 by an adhesive (see, for example, Japanese laid-open patent publication No. H11-040109, paragraph [0023] and Japanese laid-open patent publication No. 2003-229092, paragraphs [0027], [0032] through [0035], [0037], [0043], FIG. 3).
Japanese laid-open patent publication No. H11-040109 also discloses the use of a thin metal film as the external electrodes. The thin metal film is formed by a physical deposition process such as sputtering or vacuum evaporation. This publication also reveals a process of coating the outer surface of the glass bulb with an electrically conductive paste and drying the electrically conductive paste into external electrodes. Printing or dipping is used to coat the glass bulb with the electrically conductive paste.
Each of the external electrodes disclosed in Japanese laid-open patent publication No. H11-040109 is in the form of an electrically conductive thin film. Other known external electrodes in the form of an electrically conductive thin film are formed as a plated metal layer or formed by winding a metal foil around a glass bulb. It is also known to fabricate a metal tape by coating one surface of a metal foil with a tackiness agent or an adhesive, and wind the metal tape around a glass bulb.
External-electrode discharge lamps for use as the backlight for a large-screen liquid crystal display panel are required to produce high luminance, as described above. Therefore, a large discharge current necessarily flows in the external-electrode discharge lamps, and a large amount of heat is inevitably produced thereby. Since the electrically conductive thin film used as the external electrodes have a limited heat radiation capability, a vicious cycle occurs which is successively made up of an increase in the temperature of the glass bulb, a reduction in the insulation of the glass bulb, an increase in the current, an increase in the amount of generated heat, and a further increase in the temperature of the glass bulb. As a consequence, holes tend to be formed in the glass bulb beneath the external electrodes.
An electrically conductive thin film for use as an external electrode is likely to be fabricated according to a complex production process and hence at a high cost. If a thin metal film deposited by sputtering or vacuum evaporation is formed as an external electrode, then a mask needs to be formed in a shape depending on the shape of the external electrode, or the thin metal film needs to be etched after it has been grown. If an electrically conductive paste is employed, then it has to be applied and then dried. If a metal foil is bonded by a tackiness agent or an adhesive, then a processing step is required to apply the tackiness agent or the adhesive to only a required area. If a metal foil is to be wound around a glass bulb or a metal tape formed by coating one surface of a metal foil with a tackiness agent or an adhesive is to be wound around a glass bulb, then a processing step of winding the metal foil or the metal tape is required.
It has been proposed to place a ring formed from a relatively thick member such as a metal plate over a glass bulb from one end thereof for use as an external electrode. The use of such a ring as an external electrode simplifies the process of forming an external electrode. Further, the external electrode is considerably thick so that it has a better heat radiating capability. Examples of a discharge lamp having a ring formed from a metal plate are disclosed in Japanese laid-open patent publication No. 2003-017005, paragraphs [0003], [0030] through [0032], FIGS. 9 and 11, Japanese laid-open patent publication No. H8-273625, paragraphs [0013] through [0015], FIG. 1, and Japanese laid-open patent publication No. 2004-079267, paragraphs [0016] through [0018], FIGS. 1 and 2.
The discharge lamp disclosed in Japanese laid-open patent publication No. 2003-017005 has a metal conductor having spring resiliency and a C-shaped cross section, which is fitted over a glass bulb axially from one end thereof for use as an external electrode. According to Japanese laid-open patent publication No. H8-273625, a resilient metal plate such as of phosphor bronze is machined into a C-shaped cross section, and fitted over a glass bulb axially from one end thereof for use as an external electrode. The relatively thick metal plate used as the external electrode has a better heat radiating capability. Furthermore, the discharge lamp can efficiently be manufactured because the external electrode can be produced simply by fitting the C-shaped member, formed from the metal plate having spring resiliency, over the glass bulb from one end thereof.
However, simply fitting the C-shaped metal ring over the glass bulb tends to cause irregular contact between the glass bulb and the ring. As a consequence, the glass bulb and the external electrode are not held in full effective surface-to-surface contact with each other, but held in contact with each other through a small effective area, resulting in a reduced amount of electric power applied to the discharge lamp. In addition, a discharge current is liable to concentrate in the area where the contact resistance is smaller. The area where the discharge current concentrates is thus trapped in a vicious cycle that is successively made up of an increase in the amount of generated heat, a reduction in the insulation of the glass bulb, a further increase in the current, and a further increase in the amount of generated heat. As a consequence, holes tend to be formed in the glass bulb beneath the external electrode.
Japanese laid-open patent publication No. 2003-017005 and Japanese laid-open patent publication No. H8-273625 disclose a technique to improve irregular contact between a glass bulb and an external electrode when a C-shaped metal ring is used as the external electrode. Specifically, a plastic electrically conductive member is wound on the outer surface of the glass bulb which will be positioned beneath the external electrode. The external electrode in the form of the C-shaped metal ring is fitted over the plastic electrically conductive member, thereby improving the irregular contact between the glass bulb and the external electrode. The plastic electrically conductive member which serves as a base for the external electrode comprises either a metal tape in the form of a metal foil whose reverse side is coated with a tackiness agent or a silver paste. With the disclosed arrangement, though the manufacturing process is complex and the manufacturing cost is high, the irregular contact between the glass bulb and the external electrode can be improved.
Japanese laid-open patent publication No. 2004-079267 discloses a technique to bond an external electrode in the form of a C-shaped metal ring to a glass bulb with an electrically conductive adhesive. Though the disclosed technique is not directly aimed at improving irregular contact between the external electrode and the glass bulb, but is expected to improve such irregular contact between the external electrode and the glass bulb.
For achieving higher luminance, i.e., a higher discharge current, for the above conventional external-electrode discharge lamps, the external electrode should preferably comprise a relatively thick metal member, rather than a thin film such as a metal foil, a metal layer deposited by sputtering or vacuum evaporation, an electrically conductive paste coating, or a plated metal layer. Particularly, it is preferable, also from the standpoint of easy production, to machine a metal plate into a C-shaped ring having spring resiliency for use as an external electrode and insert a glass bulb into the C-shaped ring to place the external electrode on the glass bulb.
One problem with the C-shaped external electrode is that light tends to leak from the gap in the C shape of the external electrode. The light leakage is fatal if the discharge lamp is used as the light source of a backlight for liquid crystal display devices because light is emitted from the edge of the liquid crystal display panel which should not be light-emitting. In addition, the radiation emitted from the gap of the external electrode, which is radiated not through the fluorescent layer of the lamp, contains many ultraviolet rays that are liable to deteriorate plastic members such as light guide plates.
Another problem is that the area of the external electrode is reduced because of the gap in the C shape thereof. If the same discharge current flows, i.e., if the same luminance is produced, the current density is greater in the external electrode and the amount of generated heat is greater with the gap than in the external electrode with no gap. However, since the area of the electrode is smaller, the heat radiating area is smaller, thereby promoting the heating of the lamp.
The structure in which the C-shaped external electrode is mounted on the glass bulb while pressing the glass bulb under its spring resiliency tends to introduce irregularities into the contact between the glass bulb and the external electrode. As described above, the irregular contact between the glass bulb and the external electrode is liable to form holes in the portion of the glass bulb beneath the external electrode.
If a C-shaped external electrode is placed over a base comprising a metal foil coated with a tackiness agent or an adhesive or a plastic electrically conductive member made of an electrically conductive tackiness agent such as a silver paste, then the contact between the glass bulb and the external electrode is more improved than if no base is used. In addition, this structure also eliminates the problem of light leakage from the gap in the C shape. However, if a metal foil is disposed on the surface of the plastic electrically conductive member as the base in the above structure, then the base and the external electrode are simply held in mechanical contact with each other. Consequently, this structure does not fully solve the problem of the irregular contact.
If the structure in which the C-shaped external electrode is fixed to the glass bulb by the electrically conductive adhesive is used for a long period of time, close contact will be lost between the external electrode and the glass bulb. The electrically conductive adhesive comprises an electrically conductive filler such as silver particles or nickel particles which are mixed with and dispersed in an organic binder such as an epoxy resin. If the electrically conductive adhesive is used to secure the external electrode to the glass bulb, the binder resin is deteriorated with time by ultraviolet rays radiated through the glass bulb. Consequently, the contact between the external electrode and the glass bulb is impaired with time. Some organic resins, such as a silicone resin, for example, are relatively highly resistant to ultraviolet rays. However, such organic resins have low bonding strength and do no provide sufficiently reliable contact between the external electrode and the glass bulb.